The invention relates to a process for the preparation of bleaching granules containing a solid, water-insoluble organic peroxy acid, a hydratable inorganic material, and, optionally, a water-insoluble organic compound and a surface-active compound. The bleaching agents according to the invention may be used alone or as additives in all the usual fields of application for bleaching agents. Preferably, they are employed in detergent and bleaching compositions for textile laundering processes.
The chemical instability of the peroxy acid, which in the pure form is liable to exothermic decomposition, requires that special steps be taken for the preparation of bleaching granules. In addition to the chemical stability of the peroxy acid, which is enhanced by preparing bleaching granules to a level sufficient for prolonged storage, the bleaching granules, especially when used in the detergents industry, must be more or less dust-free, display a favourable solubility in water, and form free-flowing mixtures, preferably with a granule size distribution of about 0.1-5 mm.
A known method for stabilizing peroxy acids is the addition of a hydratable inorganic material which, by taking up the crystallization water at a temperature below the hydration temperature of the hydratable material, will protect the hydratable material from moisture, but will also, upon reaching critical temperatures for the decomposition of the peroxy acid, which are above the hydration temperature of the hydratable material, release hydration water and inhibit exothermic decomposition. On the other hand, the water content of the granules should not be too high, since this would affect the mechanical stability of the granules.
In DE-C-28 05 128 a process is described for the preparation of bleaching granules containing a non-hydratable peroxy acid material and a hydratable material, in which a water-wet, plastic composition is prepared at a temperature above the hydration temperature of the hydratable material. This composition is extruded into smaller units, which units are chilled, resulting in the hydration of the hydratable material, and then subjected to a subsequent drying step. The uptake of the hydration water by the hydratable material as a result of the rapid decrease in temperature leads to the fixation of the formed granules. This four-step process makes it possible to prepare stable, free-flowing bleaching granules. In actual practice, however, such a process is found to be very time-consuming and costly, since the successive steps of heating, size reduction, rapid chilling, and drying are energy intensive and different equipment is needed for each individual step. Moreover, the process requires a relatively large amount of water to form the plastic composition.
Described in EP-A-200 163 is another method for the preparation of bleaching granules which, in addition to a organic peroxy acid and a hydratable inorganic salt, contain an alkali-soluble organic polymer compound as granulation aid. For the preparation of the granules, granulation processes are mentioned which do not employ strong mechanical and thermal loads that might lead to decomposition of the peracid. The granulate may be prepared by accretion granulation in a mixing granulation process, in which the solid peracid or a peracid premix is premixed with the remaining constituents in a mixer, whereupon water or an aqueous solution of the granulation aid is introduced and stirred until the desired granule size distribution has been obtained. This mixing granulation process is carried out at a temperature in the range of ambient temperature to 45.degree. C. However, there is no teaching of the temperature rise scheme of the present invention. If needed, there may be a subsequent drying step. The mechanical stability of the granules obtained by this process is attributed to the polymeric granulation aid.
The present invention has for its object to develop a cost and energy-saving process for the preparation of bleaching granules having a very low water content, which contain at least a water-insoluble peroxy acid compound and a hydratable material and which are mechanically stable, chemically stable and free-flowing. The granulates of the present invention do not need to include a polymeric granulation aid.
The bleaching granules prepared by the process according to the invention display excellent mechanical and chemical stability, prolonged storage stability, are dust-free, soluble in water, have a low water content, and exhibit a controllable granule size distribution. The bleaching granules prepared by the process according to the invention have higher densities than the known bleaching granules, a characteristic which is becoming increasingly important to the detergent and bleaching agent industries.
The favourable mechanical stability of the granules obtained according to the invention is highly surprising in that in the present process, since no use is made of the polymeric granulation aid according to EP-A-200 163 or the rapid chilling step in accordance with DE 28 05 128.
Drying steps may either be omitted or, optionally, such drying process may be employed if low water content bleaching granules are needed. The mechanical stability of the granules is also of advantage here due to restricted attrition and a higher product yield during the drying process.